In dry climates and in climates where only heating is required it is often necessary to add moisture to the air. Air that is too dry has an adverse effect on one's health through drying of membranes, etc. It also tends to break the joints on furniture and cause various aging effects on a home. Accordingly, it is well known that it is desireable to add moisture or to humidify the air. In hot air heating systems, it is common to humidify the air in the plenum of the furnace for circulation throughout the entire building.
With other types of heating systems, such as hot water, or with heating on a room-by-room basis it is not possible to humidify the air in a furnace plenum because there is none. Individual room humidifiers are known, but they are unsightly, and they take up space in the room which could be used for other purposes.
In very dry climates, humidification could have an additional advantage of cooling the air, since evaporating water in the air produces a temperature drop.